Think in terms of eighths: .125 is one-eighth. .250 is two eighths, .375 is three-eighths, .500 is four eighths, or one half. .625 is five eighths, .750 is six-eighths, .875 is seven eighths, 1.00 is the whole.

Adding .125 each time gives you eighth-part portions. These numbers are used frequently in mortgage rate and interest financing...